One of the problems encountered in the treatment of automotive exhaust gas, and in particular, exhaust gas from lean burn engine such as diesel exhaust gas and lean burn gasoline engines, concerns the treatment of nitrous oxides contained therein. To this effect, many exhaust gas treatment systems employed in automotive vehicles running under lean burn conditions incorporate a combination of a NOx storage catalyst located upstream of an SCR catalyst. In particular, nitrogen oxide contained in an exhaust gas stream is stored at lower temperatures in the NOx storage catalyst, to be released at higher operation temperature at which an effective reduction thereof in the SCR unit may be achieved. To this extent, it is usually necessary to employ a means of injecting a reducing agent into the gas stream downstream of the NOx storage catalyst and before the SCR unit for enabling the SCR reaction of nitrogen oxide to nitrogen. For achieving an optimal SCR activity, i.e. for keeping emissions of nitrogen oxides and/or of reducing agents such as ammonia and/or urea as low as possible, numerous solutions have been proposed to coordinate the activities of the individual components of an NOx storage catalyst, a reducing agent injection means located downstream thereof, and an SCR unit for reaction of the reducing agent and the nitrogen oxide under ideal stoichiometry depending on the inlet temperature.
DE 100 11 612 A1, for example, discloses an exhaust gas treatment system for combustion engines containing a sequence of an NOx storage catalyst, a urea injection unit, and an SCR catalyst provided in the exhaust gas conduit, wherein a system of sensors regulate the injection of urea depending on the oxygen content of the exhaust gas. WO 2008/022751, on the other hand, discloses an exhaust gas treatment system having the same sequence of components, wherein the infection of ammonia upstream of the SCR unit is specifically controlled in dependency of the exhaust gas inlet temperature.
A major drawback with respect to such exhaust gas treatment systems concerns the pronounced dependency on a means for introducing a reducing agent into the exhaust gas stream, which necessarily involve high precision apparatus for both permitting the right amount of a reducing agent to be introduced into the exhaust gas at the right time for allowing an optimal conversion thereof with nitrogen oxide to nitrogen in an SCR unit. In particular, such systems require a regular maintenance for optimal functioning, and are sensitive to weather conditions, especially in the winter, when low temperatures may severally impede a proper operation mode. As a result, efforts have been made to provide exhaust treatment systems which do not display a dependency as pronounced as in the aforementioned systems, of which are even largely independent from the use of an external source of a reduction agent for treating nitrogen oxide emissions in an SCR unit.
In this respect, WO 2009/134831 discloses an exhaust treatment system for the treatment of nitrogen oxide emissions which contains a combination of a three-way catalytic device located upstream of an SCR unit, and which does not contain an external means of introducing a reducing agent into the treatment system. In particular said system relies on the in situ generation of ammonia in the three-way catalytic device, wherein the generation thereof is adapted to the ammonia needs of the SCR unit by controlling the fuel injection means such that the composition of the exhaust gas stream, and thus the stoichiometry of the components necessary for the production of ammonia on the three-way catalyst is adapted accordingly. DE 101 52 187, on the other hand, provides an alternative route by transferring the stoichiometric control of ammonia in dependency of the nitrogen oxide content in the exhaust gas stream to the exhaust gas system itself, by using a combination of nitrogen oxide and ammonia traps located therein, and controlling the exhaust gas flow by a vent system which regulates the stream in a series of parallel channels.
Independently of the manner in which in situ ammonia generation is achieved in such systems, there remains an ongoing need for improved three-way catalysts for the generation of ammonia. In particular, there is a need for catalysts which display a high activity with respect to the formation of ammonia in rich exhaust gas phases for reducing the necessity and the duration of said phases in view of further reducing total exhaust emissions. Furthermore, a challenging problem in the development of such improved catalysts relates to the fact that the improvement in ammonia yield usually goes hand in hand with a worsening of the three-way catalyst's further functions, in particular regarding its oxidation capacity with respect to hydrocarbons.